Some oxovanadium(IV) complexes, namely bis(1,1,1-trifluro-2,4-pentanedionato-O,O′) oxovanadium (IV), [VO(tfac)2(H2O)], bis(1-phenyl-2,4-pentanedionato-O,O′)oxovanadium(IV), [VO (phac)2(H2O)], bis(1,3-diphenyl-2,4-pentanedionato-O,O′)oxovanadium(IV), [VO(dphac)2 (H2O)], of the type [VO(O4)] and bis(pyrolidineaniline)oxovanadium(IV), [VO(pyran)2(H2O)], bis(p-hydroxypyrolidineaniline) oxovanadium(IV), [VO(p-hydroxypyran)2(H2O)], bis(p-methoxypyrolidineaniline) oxovanadium(IV), [VO(p-MeOpyran)2 (H2O)], bis(p-chloropyrolidineaniline)oxovanadium(IV), [VO(pchloropyran)2(H2O)], bis(p-bromopyrolidineaniline)oxovanadium(IV), [VO(p-bromopyran)2(H2O)], bis(p-cyano pyrolidineaniline)oxovanadium(IV), [VO(p-cyanopyran)2(H2O)], and bis(pyrolidinebenzylamine) oxovanadium(IV), [VO(pyrbz)2(H2O)], of the type [VO(N4)] were synthesized and characterized by IR, UV–Vis, mass spectrometry, elemental analysis, magnetic moment and thermogravimetry in order to evaluate their thermal stability and thermal decomposition pathways. The number of steps and, in particular, the starting temperature of decomposition of these complexes depends on the equatorial ligand. Also, formation constants of the complexes have been determined by UV-Vis absorption spectroscopy through titration of the ligands with the metal ions at constant ionic strength (0⋅1 M NaClO4) and at 25°C. According to the thermodynamic studies, as the steric character of the ligand increases, the complexation tendency to VO(IV) center decreases. Also, the ab initio calculations were carried out to determine the structural and the geometrical properties of the complexes.

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